Understanding Datum Features and Reference Frames in Engineering Drawings

2. 3 Datums

3. L earning Objectives • Explain the purpose of datums. • Define common terms related to datum features. • Interpret information provided by datum feature symbols. • Describe methods for representing datum surfaces on drawings. • Explain how to establish datum reference frames.

4. L earning Objectives • Identify the degrees of freedom of a part. • Explain how to establish datum targets. • Identify datum target points, areas, and lines. • Describe methods for representing datum axes on drawings. • Describe methods for representing datum center planes on drawings.

5. Datums Represent true geometric counterpart of a datum feature Reference features of an object Establish location and size dimensions

6. Datum Feature Simulator • Opposite shape of datum feature • Two types: • Theoretical datum feature simulator • Physical datum feature simulator • Manufacturing examples: • Machine tables • Surface plates • Gauge surfaces • Surface tables • Specially designed rotation devices

7. Datum Feature Symbol Do not apply to centerlines, center planes, or axes Draw using thin lines Use any letter to identify a datum feature except I, O, or Q Use double letters when number of datums exceeds number of letters in alphabet

8. Datum Feature Symbol

9. Datum Feature Datum plane Part Datum feature Simulated datum plane Physical datum feature simulator Surface of manufacturing or verification equipment

10. Datum Feature Terms Actual mating envelope Simulated datum Tangent plane

11. Datum Feature Symbol Placement for Surfaces

12. Datum Surface Can be controlled by geometric tolerance Datum plane measurements do not take variations in datum surface from datum plane into account

13. Geometric Control of Datum Surface

14. Datum Reference Frame (DRF) Concept • Select three perpendicular datum features based on their importance to part design • Assign precedence and datum reference order • Primary datum • Secondary datum • Tertiary datum

15. Datum Reference Frame Concept

16. Datum Reference Frame Concept: Inspecting the Part • All parts have six degrees of freedom • Three degrees of translation • Three degrees of rotation • Movement is either translational or rotational

17. Degrees of Freedom

18. Establishing Multiple Datum Reference Frames Datums X, Y, and Z establish one datum reference frame Datums L and M establish second datum reference frame

19. Datum Features Specified Individually Place a note next to datum feature symbols indicating how many datum features to consider separately Place the note “2X INDIVIDUALLY” next to datum feature symbols of two separate datum features identified by same letter

20. Datum Target Symbols Identify datum targets Useful on parts with surface or contour irregularities Connect to datum target point, line, or area with a leader

21. Datum Target Symbols

22. Datum Target Points Establish primary datum plane by locating at least three points on primary datum surface Establish secondary datum plane by locating at least two points on related secondary datum surface Establish tertiary datum plane by locating at least one point on related tertiary datum surface Dimension using baseline or chain dimensioning Location dimensions originate from datums

23. Datum Target Points

24. Datum Target Points Datum plane established by locating pins at datum target points

25. Datum Target Areas Used to establish datums using areas of contact Locating pins are flat end tooling pins with pin diameter equal to specified size of target area Top half of the datum target symbol identifies size of target area Use datum target point at center location when area is too small to accurately or clearly display on drawing

26. Datum Target Areas

27. Datum Target Lines Indicated by a target point on edge view of surface and a phantom line on surface view Locating pins are cylindrical or knife-edged with datum target line along tangency where pins meet the part Surface is often placed 90 to the pin to create datum reference frame

28. Datum Target Lines

29. Partial Datum Surface

30. Coplanar Surface Datums Used to establish coplanar surfaces as one datum plane Specify number of surfaces with a note such as: "2 SURFACES" below related feature control frame

31. Coplanar Surface Datums

32. Datum Axis • Represents two theoretical planes intersecting at 90 • Represented in drawings with centerlines • Pitch cylinder for screw threads establishes datum axis unless otherwise specified • When not using pitch cylinder for screw threads, place note such as "MAJOR DIA" or "MINOR DIA" next to datum feature symbol • Simulated datum axis established by inspection equipment

33. Datum Axis

34. Datum Feature Symbol Placement for a Datum Axis

35. Coaxial Datum Features Separate datum reference letters by a dash in one compartment of the feature control frame

36. Datum Axis Established with Datum Target Symbols Primary datum axis established by two sets of three equally spaced targets Identify datum target points in correlation to adjacent cylindrical datum feature when two cylindrical features of different diameters establish a datum axis

37. Datum Axis Established with Datum Target Symbols • Cylindrical datum target areas and circular datum target lines can be used to establish datum axis • Target area represented by two phantom lines with section lines between • Datum target line represented by phantom line all around part • Establish secondary datum axis by placing three equally spaced targets on cylindrical surface

38. Movable Datum Target Symbols and Datum Target Points When datum targets establish a center point, axis, or center plane on a RMB basis, datum feature simulator movement is normal to true profile

39. Movable Datum Target Symbols and Datum Target Spheres Circular phantom line represents spherical datum feature simulator to establish datum axis

40. Datum Center Plane Axis and center plane datum feature symbols align with/replace dimension line arrowhead or appear on feature, leader shoulder, dimension line, or feature control frame

41. Datum Center Plane

42. Translation Modifier Added to feature control frame following datum feature reference and any other applicable modifiers Specifies that basic location of datum feature simulator is unlocked and free to translate within specified geometric tolerance

43. Translation Modifier

44. Using Contoured Surface as Datum Feature 3D mathematical coordinate system defines compound surface feature on a part Datum simulator results from mathematical data Common when design copies contour of an actual object that has few or no flat surfaces to use as datums

45. R eview Identify the datum feature, part, simulated datum plane, physical datum feature simulator, and datum plane. A. Datum feature B. Datum plane C. Part D. Simulated datum plane E. Physical datum feature simulator

46. R eview Identify Parts A and B. A. Datum feature symbol B. Feature control frame

47. R eview Identify parts A and D and provide dimensions for B, C, and D. A. Datum plane A B. 12.2 minimum C. 12.8 minimum D. 0.1 geometric tolerance

48. R eview Identify items A–G. A. Basic dimension B. Datum feature symbol C. Target point D. Datum target symbol E. Datum reference F. Target number G. Datum target line

49. R eview Identify items A–C. A. Datum target area B. Datum target symbol C. Target area size

50. R eview Identify whether the datum reference symbol represents a datum surface, datum axis, or datum center plane in items A–C. A. Datum surface B. Datum center plane C. Datum axis